In a practical world of revenue targets, it can be difficult to always slow down long enough to stop and test and then carefully analyze your results.
Understanding the validity of your data can help you to quickly make decisions and truly understand what a test is telling you.
Simply put, if you have a larger variance between two results, then you will need a smaller sample size to achieve a strong degree of confidence.
Go here for info on how to calculate validity:
www.marketingexperiments.com/validity.html
Happy Testing!!!